1. Field of the Invention
The present invention relates to a touch panel, and more particularly to a projected capacitive touch panel effectively enlarging the size of the touch panel and preventing signal transmission lines thereon in the X and Y directions from being overlong.
2. Description of the Related Art
Current touch panels are classified as capacitive touch panels and resistive touch panels. For sake of lower cost, the resistive touch panels have larger market share of all sorts of touch panels. Since Apple's iPHONE emergence to the touch panel market, in addition to the wide range of touch control, the multi-point touch function has become one of the best selling points. However, the multi-point touch feature originates from the projected capacitive touch panels, which also pertain to one type of capacitive touch panels.
With reference to FIG. 6, a conventional projected capacitive touch panel has a substrate 70, an X-axis electrode layer 71 and a Y-axis electrode layer 72. The substrate 70 is transparent and has a top and a bottom. The X-axis electrode layer 71 is mounted on the top of the substrate 70 and has a plurality of X-axis electrodes 711. With reference to FIG. 7, the X-axis electrodes 711 are rhombic and align in a matrix form. The X-axis electrodes 711 on a same row are mutually connected, and the X-axis electrode 711 located on one end of each row is further connected with an X-axis signal transmission line 712. The Y-axis electrode layer 72 is mounted on the bottom of the substrate 70 and has a plurality of Y-axis electrodes 721. The Y-axis electrodes 721 are rhombic, align in a matrix form and alternately and crossly intersect with the X-axis electrodes 711. The Y axis electrodes 721 on a same column are mutually connected, and the Y-axis electrode 721 located on one end of each column is further connected with a Y-axis signal transmission line 722.
The X-axis signal transmission lines 712 and the Y-axis signal transmission lines 722 are normally mounted on the substrate and alongside a side of the substrate 70 and are concentrated and connected with a flexible printed circuit board (PCB). The wires on the flexible PCB are further connected with a control circuit.
With reference to FIG. 8, a touch panel 80 of a regular smart phone is shown. The touch panel 80 has a plurality of X-axis signal transmission lines and a plurality of Y-signal transmission lines. The X-axis signal transmission lines and the Y-axis signal transmission lines are concentrated on a place adjacent to a side of the touch panel 80 and are connected with a flexible PCB 81. The flexible PCB 81 has an analog-to-digital (A/D) converter 82 and a controller 83 mounted on the flexible PCB by a chip on flexible printed circuit (COF) technique. The A/D converter 82 has a plurality of input terminals respectively connected with the X-axis signal transmission lines and the Y-axis signal transmission lines through wires on the flexible PCB 81 to convert analog signals sent from the X-axis signal transmission lines and the Y-axis signal transmission lines into digital signals and further transmit the digital signals to a controller 83. The controller 83 determines x and y coordinates being touched on the touch panel 80.
With reference to FIG. 9, a touch panel that is slightly larger has a plurality of X-axis signal transmission lines and a plurality of Y-axis signal transmission lines concentrated on a lower portion of the touch panel 90, connected with two flexible PCBs and electrically connected with a PCB 93 through the two flexible PCBs 91, 92. The PCB 93 has at least one A/D converter and a controller. The at least one A/D converter is respectively connected with the X-axis signal transmission lines and Y-axis signal transmission lines through the flexible PCBs 91, 92, converts received analog signals into digital signals and sends the digital signal to the controller. The controller computes and determines X and Y coordinates touched on the touch panel in accordance with the received digital signals.
The A/D converter and the controller of the conventional projected capacitive touch panels are mounted to be adjacent to proximal ends of the X-axis signal transmission lines and the Y-axis signal transmission lines. For a compact projected capacitive touch panel, such approach impacts little upon signal stability of the X-axis signal transmission lines and the Y-axis signal transmission lines. However, for a large projected capacitive touch panel, the X-axis signal transmission lines and the Y-axis signal transmission lines must be commonly connected to a circuit board. As a result, the X-axis signal transmission lines and the Y-axis signal transmission lines illustrated in FIG. 8. must be routed and concentrated on one side of the side frame of the touch panel, meaning that the larger the size of the touch panel is, the longer the X-axis signal transmission lines and the Y-axis signal transmission lines are. Usually, the longer the signal transmission distance is, the lower the capability against interference is. Furthermore, as the X-axis signal transmission lines and the Y-axis signal transmission lines must be commonly connected to a circuit board and concentrated on one side of the touch panel, the width of the side frame of the touch panel for accommodating the layout of the X-axis signal transmission lines and the Y-axis signal transmission lines cannot be reduced. Such issue becomes an obstacle in further enlarging the size of a projected capacitive touch panel.
To sum up, the size of conventional projected capacitive touch panels fails to be significantly enlarged due to the limitations of the lengths and wiring layout of the X-axis signal transmission lines and the Y-axis signal transmission lines. A feasible solution needs to be addressed to refine the aforementioned issue.